As one of measuring instruments for measuring a flow rate of a fluid flowing through a pipe, a differential pressure detector is conventionally used and this differential pressure detector constitutes a restriction flowmeter which allows an orifice or venturi tube, etc., having a restriction portion to measure a hydrostatic differential pressure using a manometer and which enables a flow velocity or flow rate to be measured.
Such a conventional differential pressure detector requires precise machining as specified in the Japanese Industrial Standard and its measuring accuracy decreases when the flow is disturbed, and therefore appropriate straight tubes need to be provided upstream and downstream from installed equipment. Furthermore, a differential pressure generated from the differential pressure detector is expressed by only one curve as a square equation of the flow velocity or flow rate of the fluid flowing through the pipe.
Moreover, when a fluid to be measured is liquid, erosion with use for an extended period of time causes wear in the edge or deposition of sludge before and after the restriction portion causes deterioration of measuring accuracy.
However, the above described background art requires precise machining, hence a cost increase, which produces some cases where installation of the differential pressure detector is passed up despite the necessity for flow rate measurements. Especially consumer buildings which are most behind in a CO2 reduction, an environmental issue gathering attention nowadays, are required to exercise energy-saving control over heat sinks, but differential pressure detectors are hardly installed for reasons related to initial costs.
Moreover, a differential pressure detector based on the conventional art needs to be installed in a place where the flow of a fluid through the pipe is rectified so as to maintain the measuring accuracy. When it is installed, for example, after a local resistance section such as an R elbow, quite a long straight pipe needs to be provided, and therefore it is hard to use the differential pressure detector because of constraints in a place where it is actually used.
Furthermore, according to the differential pressure detector based on the conventional art, the relationship between a differential pressure generated and the flow velocity or flow rate of a fluid is expressed by a square curve, and therefore when measurements are made from a low flow velocity area to high flow velocity area using a single manometer, the range of the differential pressure generated is wide and a large measuring error may occur in the low flow velocity area depending on the performance of the manometer. Furthermore, there is a problem that use for an extended period of time causes wear in the edge or deposition of sludge and causes deterioration of measuring accuracy.
Furthermore, when a Pitot tube is used as a conventional differential pressure detector, a total pressure and hydrostatic pressure are extracted and a differential pressure between them is obtained, but this can only obtain local velocities at measuring points and in order to calculate an average velocity, it is necessary to measure velocities at several points within the same cross section.
Furthermore, as disclosed in JP-A-8-210887, a tube member through which a fluid flows is provided with a total pressure detection cylinder member penetrating in a direction perpendicular to the axis of the tube member and this total pressure detection cylinder member is provided with a total pressure detection hole opened facing the flowing direction of the fluid and the wall of the tube member is provided with an opening so as to detect a hydrostatic pressure.
However, while this differential pressure detector can detect at a plurality of points on the same cross section, it cannot detect a large differential pressure in a low flow velocity area.
The present invention has been implemented in view of the above described problems and it is an object of the present invention to provide a restriction flowmeter, which is a low-priced product without requiring precise machining, capable of measuring with a high degree of accuracy even under disturbed flow conditions, expressing the relationship between a differential pressure generated from the differential pressure detector, flow velocity and flow rate as a plurality of square curves and maintaining stable measuring accuracy for a long period of time.